Notable

Private & encrypted rich-text documents. Cryptee has encryption and anonymity at its core, it also has a beautiful and minimalistic UI. You can use Cryptee from the browser, or download native apps. Comes with many additional features, such as support for photo albums and file storage. The disadvantage is that only the frontend is open source. Pricing is free for starter plan, $3/ month for 10GB, additional plans go up-to 2TB.

Cross-platform desktop and mobile note-taking and todo app. Easy organisation into notebooks and sections, revision history and a simple UI. Allows for easy import and export of notes to or from other services. Supports synchronisation with cloud services, implemented with E2EE.

Privacy-first, open-source knowledge base that works on top of local plain-text Markdown and Org-mode files. Supports lots of different note modes, including task management, PDF annotation, flashcards, whiteboards strong markdown support and more. Includes themes and extensions, backed by a strong community

A powerful knowledge base that works on top of local plain-text Markdown files. It has a strong community, and a lot of plugins and themes. Generally privacy-respecting, but no encryption out of the box, and some of the code is obfuscated or not fully open source

Self-hosted notes and tasks app with rich-text, Markdown and drawing editors, tags, multi-user, OIDC login and a REST API. Docker-based (PHP + SQLite); the only outbound call is a daily update check. No mobile apps (PWA), no end-to-end encryption.

S.Notes is a free, open-source, and completely encrypted private notes app. It has a simple UI, yet packs in a lot of features, thanks to the Extensions Store, allowing for: To-Do lists, Spreadsheets, Rich Text, Markdown, Math Editor, Code Editor and many more. You can choose between a number of themes (yay, dark mode!), and it features built-in secure file store, tags/ folders, fast search and more. Standard Notes is actively developed, and fully open-source.

A secure, collaborative notebook. Self-host it yourself, or use their hosted plan (free edition or $3/ month for premium).

A free, open-source note-taking application built with Qt, focused on providing a pleasant Markdown editing experience. It manages notes directly as plain text files on your local system.